Richard Harris gave the rescue mission the final nod of approval.

Around 20 Australians have been involved in rescue efforts to free 12 boys and their soccer coach from a Thai cave.

Key among them is Dr. Richard Harris, a diver who has experienced the grief of a treacherous cave system before.

Beneath the earth in South Australia's south-east is our nation's most complex known cave system, Tank Cave, a series of passageways about seven kilometers in length.

Complex and narrow, the system is not dissimilar to the caves being navigated by rescue divers in Thailand. It's the experience of Tank Cave that Harris took with him when he was called this week to Chiang Rai, to assist in the soccer team rescue mission.

Also a doctor who works with South Australia's MedStar emergency response
group, 53-year-old Harris has 30 years of cave diving experience.

Prior to this week, it was in Tank Cave that he faced one of his greatest professional and personal challenges.

In February 2011, Harris' friend Agnes, also an experienced cave diver, perished in the dark and narrow underwater passageways near Mount Gambier.

It was Harris' job to go in first and find the body of the 29-year-old. Agnes had become separated from her diving partner, disorientated and ultimately ran out of air in a tight corridor of rock 20 metres underwater. Harris spent two days diving in the caves, clearing a path for her body to be retrieved.

Thai authorities sent Harris into the Tham Luang cave to swim to the trapped boys immediately upon his arrival at the site on Saturday.

It was Harris who gave the medical all-clear for rescue efforts to begin. Now four boys have been safely freed from the cave, while eight more and their soccer coach remain trapped after 16 days of isolation.

The first of the Wild Boars soccer team emerged from the cave at 5.37pm local time. The second, 13 minutes later and the third following another 16 minutes. The four boys survived the treacherous journey through the Tham Luang caves, all emerging to walk unscathed into the hands of waiting medics.

Six Australians were among the 18 divers involved in the rescue band. Now, having used all the oxygen supplies, they'll need between 10 and 20 hours to resupply before resuming the rescue mission.